Earth
From Wikipedia, the free encyclopedia
Earth, also known as
the world,
[25] Terra,
[27] or
Gaia,
[29] is the third
planet from the
Sun, the
densest planet in the
Solar System, the largest of the Solar System's four
terrestrial planets, and the only
celestial body known to accommodate
life. It is home to millions of
species,
[30] including
billions of
humans[31] who depend upon its
biosphere and
minerals. The Earth's human population is divided among
about two hundred independent states that interact through diplomacy, conflict, travel, trade, and
media.
According to evidence from sources such as
radiometric dating, Earth was formed
around four and a half billion years ago. Within its first billion years,
[32] life appeared in its oceans and began to affect
its atmosphere and surface, promoting the proliferation of
aerobic as well as
anaerobic organisms and causing the formation of the atmosphere's
ozone layer. This layer and
Earth's magnetic field block the most life-threatening parts of the
Sun's radiation, so life was able to flourish on land as well as in water.
[33] Since then, Earth's
position in the Solar System,
its physical properties and
its geological history have allowed life to persist.
Earth's
lithosphere is divided into several rigid segments, or
tectonic plates, that migrate across the surface over periods of
many millions of years. Over 70% percent of Earth's surface is covered with water,
[34]
with the remainder consisting of continents and islands which together
have many lakes and other sources of water that contribute to the
hydrosphere. Earth's
poles are mostly covered with ice that is the solid ice of the
Antarctic ice sheet and the
sea ice that is the
polar ice packs.
The planet's interior remains active, with a solid iron
inner core, a liquid
outer core that generates the magnetic field, and a thick layer of relatively solid
mantle.
Earth
gravitationally interacts with other objects in space, especially the Sun and the
Moon. During one orbit around the Sun, the Earth rotates about its own axis 366.26 times, creating 365.26
solar days, or one
sidereal year.
[n 6] The Earth's axis of rotation is
tilted 23.4° away from the
perpendicular of its
orbital plane, producing seasonal variations on the planet's surface with a period of one
tropical year (365.24 solar days).
[35] The Moon is Earth's only
natural satellite. It began orbiting the Earth about
4.53 billion years ago (bya). The Moon's gravitational interaction with Earth stimulates ocean
tides, stabilizes the axial tilt, and gradually slows the planet's rotation.
Name and etymology
The modern English
Earth developed from a wide variety of
Middle English forms,
[37] which derived from an
Old English noun most often spelled
eorðe.
[36] It has cognates in every
Germanic language and their
proto-Germanic root has been reconstructed as
*erþō. In its earliest appearances,
eorðe was already being used to translate the many senses of
Latin terra and
Greek γῆ (
gē): the ground,
[39] its soil,
[41] dry land,
[44] the human
world,
[46] the surface of the world (including the sea),
[49] and the globe itself.
[51] As with
Terra and
Gaia, Earth was a
personified goddess in
Germanic paganism: the
Angles were listed by
Tacitus among the
devotees of
Nerthus[52] and later
Norse mythology included
Jörð, a giantess often given as the mother of
Thor.
[53]
Originally,
earth was written in lowercase and, from
early Middle English, its
definite sense as "the globe" was expressed as
the earth. By
early Modern English, many nouns were capitalized and
the earth became (and often remained)
the Earth, particularly when referenced along with other heavenly bodies. More recently, the name is simply given as
Earth, by analogy with the names of the
other planets.
[36] House styles now vary:
Oxford spelling
recognizes the lowercase form as the most common, with the capitalized
form an acceptable variant. Another convention capitalizes Earth when
appearing as a name (e.g., "Earth's atmosphere") but writes it in
lowercase when preceded by
the (e.g., "the atmosphere of the
earth"). It almost always appears in lowercase in colloquial expressions
such as "what on earth are you doing?"
[54]
Composition and structure
Earth is a terrestrial planet, meaning that it is a rocky body, rather than a
gas giant like
Jupiter.
It is the largest of the four terrestrial planets in size and mass. Of
these four planets, Earth also has the highest density, the highest
surface gravity, the strongest magnetic field, and fastest rotation,
[55] and is probably the only one with active
plate tectonics.
[56]
Shape
Stratocumulus clouds over the Pacific, viewed from orbit
The shape of the Earth approximates an
oblate spheroid, a sphere flattened along the axis from pole to pole such that there is a
bulge around the
equator.
[57] This bulge results from the
rotation of the Earth, and causes the diameter at the equator to be
43 km (kilometer) larger than the
pole-to-pole diameter.
[58] For this reason the furthest point on the surface from the Earth's center of mass is the
Chimborazo volcano in
Ecuador.
[59] The average diameter of the reference spheroid is about
12742 km, which is approximately 40,000 km/
π, as the
meter was originally defined as 1/10,000,000 of the distance from the equator to the
North Pole through
Paris,
France.
[60]
Local
topography deviates from this idealized spheroid, although on a global scale, these deviations are small: Earth has a
tolerance of about one part in about 584, or 0.17%, from the reference spheroid, which is less than the 0.22% tolerance allowed in
billiard balls.
[61] The largest local deviations in the rocky surface of the Earth are
Mount Everest (8,848 m above local sea level) and the
Mariana Trench (
10911 m
below local sea level). Due to the equatorial bulge, the surface
locations farthest from the center of the Earth are the summits of
Mount Chimborazo in
Ecuador and
Huascarán in
Peru.
[62][63][64]
Chemical composition
The mass of the Earth is approximately
5.98×1024 kg. It is composed mostly of
iron (32.1%),
oxygen (30.1%),
silicon (15.1%),
magnesium (13.9%),
sulfur (2.9%),
nickel (1.8%),
calcium (1.5%), and
aluminium (1.4%); with the remaining 1.2% consisting of trace amounts of other elements. Due to
mass segregation,
the core region is believed to be primarily composed of iron (88.8%),
with smaller amounts of nickel (5.8%), sulfur (4.5%), and less than 1%
trace elements.
[66]
The geochemist
F. W. Clarke calculated that a little more than 47% of the Earth's
crust
consists of oxygen. The more common rock constituents of the Earth's
crust are nearly all oxides; chlorine, sulfur and fluorine are the only
important exceptions to this and their total amount in any rock is
usually much less than 1%. The principal oxides are silica, alumina,
iron oxides, lime, magnesia, potash and soda. The silica functions
principally as an acid, forming silicates, and all the commonest
minerals of igneous rocks are of this nature. From a computation based
on 1,672 analyses of all kinds of rocks, Clarke deduced that 99.22% were
composed of 11 oxides (see the table at right), with the other
constituents occurring in minute quantities.
[67]
Internal structure
The interior of the Earth, like that of the other terrestrial planets, is divided into layers by their
chemical or physical (
rheological)
properties, but unlike the other terrestrial planets, it has a distinct
outer and inner core. The outer layer of the Earth is a chemically
distinct
silicate solid
crust, which is underlain by a highly
viscous solid mantle. The crust is separated from the mantle by the
Mohorovičić discontinuity, and the thickness of the crust varies: averaging
6 km (kilometers) under the oceans and 30-
50 km on the continents. The crust and the cold, rigid, top of the
upper mantle
are collectively known as the lithosphere, and it is of the lithosphere
that the tectonic plates are comprised. Beneath the lithosphere is the
asthenosphere,
a relatively low-viscosity layer on which the lithosphere rides.
Important changes in crystal structure within the mantle occur at 410
and
660 km below the surface, spanning a
transition zone that separates the upper and lower mantle. Beneath the mantle, an extremely low viscosity liquid
outer core lies above a solid
inner core.
[68] The inner core may rotate at a slightly higher
angular velocity than the remainder of the planet, advancing by 0.1–0.5° per year.
[69]
Geologic layers of the Earth[70]
Earth cutaway from core to exosphere. Not to scale. |
Depth[71]
km |
Component Layer |
Density
g/cm3 |
| 0–60 |
Lithosphere[n 7] |
— |
| 0–35 |
Crust[n 8] |
2.2–2.9 |
| 35–60 |
Upper mantle |
3.4–4.4 |
| 35–2890 |
Mantle |
3.4–5.6 |
| 100–700 |
Asthenosphere |
— |
| 2890–5100 |
Outer core |
9.9–12.2 |
| 5100–6378 |
Inner core |
12.8–13.1 |
Heat
Earth's
internal heat comes from a combination of
residual heat from planetary accretion (about 20%) and heat produced through
radioactive decay (80%).
[72] The major heat-producing isotopes in Earth are
potassium-40,
uranium-238,
uranium-235, and
thorium-232.
[73] At the center, the temperature may be up to 6,000 °C (10,830 °F),
[74] and the pressure could reach 360
GPa.
[75]
Because much of the heat is provided by radioactive decay, scientists
believe that early in Earth's history, before isotopes with short
half-lives had been depleted, Earth's heat production would have been
much higher. This extra heat production, twice present-day at
approximately
3 byr,
[72] would have increased temperature gradients within Earth, increasing the rates of
mantle convection and plate tectonics, and allowing the production of igneous rocks such as
komatiites that are not formed today.
[76]
Present-day major heat-producing isotopes[77]
| Isotope |
Heat release
W/kg isotope |
Half-life
years |
Mean mantle concentration
kg isotope/kg mantle |
Heat release
W/kg mantle |
| 238U |
9.46 × 10−5 |
4.47 × 109 |
30.8 × 10−9 |
2.91 × 10−12 |
| 235U |
5.69 × 10−4 |
7.04 × 108 |
0.22 × 10−9 |
1.25 × 10−13 |
| 232Th |
2.64 × 10−5 |
1.40 × 1010 |
124 × 10−9 |
3.27 × 10−12 |
| 40K |
2.92 × 10−5 |
1.25 × 109 |
36.9 × 10−9 |
1.08 × 10−12 |
The mean heat loss from Earth is
87 mW m−2, for a global heat loss of
4.42 × 1013 W.
[78] A portion of the core's thermal energy is transported toward the crust by
mantle plumes; a form of convection consisting of upwellings of higher-temperature rock. These plumes can produce
hotspots and
flood basalts.
[79]
More of the heat in Earth is lost through plate tectonics, by mantle
upwelling associated with mid-ocean ridges. The final major mode of heat
loss is through conduction through the lithosphere, the majority of
which occurs in the oceans because the crust there is much thinner than
that of the continents.
[80]
Tectonic plates
The mechanically rigid outer layer of the Earth, the lithosphere, is
broken into pieces called tectonic plates. These plates are rigid
segments that move in relation to one another at one of three types of
plate boundaries:
Convergent boundaries, at which two plates come together,
Divergent boundaries, at which two plates are pulled apart, and
Transform boundaries, in which two plates slide past one another laterally.
Earthquakes, volcanic activity,
mountain-building, and
oceanic trench formation can occur along these plate boundaries.
[82]
The tectonic plates ride on top of the asthenosphere, the solid but
less-viscous part of the upper mantle that can flow and move along with
the plates,
[83] and their motion is strongly coupled with convection patterns inside the Earth's mantle.
As the tectonic plates migrate across the planet, the ocean floor is
subducted
under the leading edges of the plates at convergent boundaries. At the
same time, the upwelling of mantle material at divergent boundaries
creates
mid-ocean ridges. The combination of these processes continually recycles the
oceanic crust back into the mantle. Due to this recycling, most of the ocean floor is less than
100 myr old in age. The oldest oceanic crust is located in the Western Pacific, and has an estimated age of about
200 myr.
[84][85] By comparison, the oldest dated continental crust is
4030 myr.
[86]
The seven major plates are the
Pacific,
North American,
Eurasian,
African,
Antarctic,
Indo-Australian, and
South American. Other notable plates include the
Arabian Plate, the
Caribbean Plate, the
Nazca Plate off the west coast of
South America and the
Scotia Plate in the southern
Atlantic Ocean. The Australian Plate fused with the Indian Plate between 50 and
55 mya. The fastest-moving plates are the oceanic plates, with the
Cocos Plate advancing at a rate of 75 mm/year
[87]
and the Pacific Plate moving 52–69 mm/year. At the other extreme, the
slowest-moving plate is the Eurasian Plate, progressing at a typical
rate of about 21 mm/year.
[88]
Surface
Features of Earth's solid surface shown as percentages of the planet's total surface area
Oceanic ridges (22.1%)
Ocean basin floors (29.8%)
Continental mountains (10.3%)
Continental lowlands (18.9%)
Continental shelves and slopes (11.4%)
Continental rise (3.8%)
Volcanic island arcs, trenches, submarine volcanoes, and hills (3.7%)
The Earth's
terrain varies greatly from place to place. About 70.8%
[13] of the surface is covered by water, with much of the
continental shelf below sea level. This equates to
361.132 million km2 (139.43 million sq mi).
[89] The submerged surface has mountainous features, including a globe-spanning
mid-ocean ridge system, as well as undersea volcanoes,
[58] oceanic trenches,
submarine canyons,
oceanic plateaus and
abyssal plains. The remaining 29.2% (
148.94 million km2, or 57.51 million sq mi) not covered by water consists of mountains, deserts, plains, plateaus, and other
geomorphologies.
The planetary surface undergoes reshaping over geological time periods due to
tectonics and erosion. The surface features built up or deformed through plate tectonics are subject to steady
weathering from
precipitation, thermal cycles, and chemical effects.
Glaciation,
coastal erosion, the build-up of
coral reefs, and large meteorite impacts
[90] also act to reshape the landscape.
The
continental crust consists of lower density material such as the
igneous rocks granite and
andesite. Less common is
basalt, a denser volcanic rock that is the primary constituent of the ocean floors.
[91] Sedimentary rock
is formed from the accumulation of sediment that becomes compacted
together. Nearly 75% of the continental surfaces are covered by
sedimentary rocks, although they form only about 5% of the crust.
[92] The third form of rock material found on Earth is
metamorphic rock,
which is created from the transformation of pre-existing rock types
through high pressures, high temperatures, or both. The most abundant
silicate minerals on the Earth's surface include
quartz, the
feldspars,
amphibole,
mica,
pyroxene and
olivine.
[93] Common carbonate minerals include
calcite (found in
limestone) and
dolomite.
[94]
The
pedosphere is the outermost layer of the Earth that is composed of soil and subject to
soil formation processes. It exists at the interface of the
lithosphere, atmosphere,
hydrosphere and biosphere. Currently the total arable land is 13.31% of the land surface, with only 4.71% supporting permanent crops.
[14] Close to 40% of the Earth's land surface is presently used for cropland and pasture, or an estimated 1.3
×10
7 km
2 of cropland and 3.4
×10
7 km
2 of pastureland.
[95]
The elevation of the land surface of the Earth varies from the low point of −418 m at the
Dead Sea, to a 2005-estimated maximum altitude of 8,848 m at the top of
Mount Everest. The mean height of land above sea level is 840 m.
[96]
Besides being divided logically into Northern and Southern
Hemispheres centered on the earths poles, the earth has been divided
arbitrarily into
Eastern and
Western Hemispheres.
The surface of the Earth is traditionally divided into seven continents
and various seas. As people settled and organized the planet, nearly
all the land was divided into nations. As of 2013, there are about 196
recognized nations.
[97] An example of how major geographical regions can be broken down is
Africa,
America,
Antarctica,
Asia,
Australia, and
Europe.
Hydrosphere
Elevation histogram of the surface of the Earth
The abundance of water on Earth's surface is a unique feature that
distinguishes the "Blue Planet" from others in the Solar System. The
Earth's hydrosphere consists chiefly of the oceans, but technically
includes all water surfaces in the world, including inland seas, lakes,
rivers, and underground waters down to a depth of 2,000 m. The deepest
underwater location is
Challenger Deep of the
Mariana Trench in the
Pacific Ocean with a depth of 10,911.4 m.
[n 10][98]
The mass of the oceans is approximately 1.35
×10
18 metric tons, or about 1/4400 of the total mass of the Earth. The oceans cover an area of
3.618×108 km2 with a mean depth of
3682 m, resulting in an estimated volume of
1.332×109 km3.
[99] If all the land on Earth were spread evenly, water would rise to an altitude of more than 2.7 km.
[n 11] About 97.5% of the water is saline, while the remaining 2.5% is fresh water. Most fresh water, about 68.7%, is currently ice.
[100]
The average
salinity of the Earth's oceans is about 35 grams of salt per kilogram of sea water (3.5% salt).
[101] Most of this salt was released from volcanic activity or extracted from cool, igneous rocks.
[102]
The oceans are also a reservoir of dissolved atmospheric gases, which
are essential for the survival of many aquatic life forms.
[103] Sea water has an important influence on the world's climate, with the oceans acting as a large
heat reservoir.
[104] Shifts in the oceanic temperature distribution can cause significant weather shifts, such as the
El Niño-Southern Oscillation.
[105]
Atmosphere
The
atmospheric pressure on the surface of the Earth averages 101.325
kPa, with a
scale height of about 8.5 km.
[3]
It is 78% nitrogen and 21% oxygen, with trace amounts of water vapor,
carbon dioxide and other gaseous molecules. The height of the
troposphere
varies with latitude, ranging between 8 km at the poles to 17 km at the
equator, with some variation resulting from weather and seasonal
factors.
[106]
Earth's biosphere has significantly altered its
atmosphere.
Oxygenic photosynthesis evolved
2.7 bya,
forming the primarily nitrogen–oxygen
atmosphere of today.
[107] This change enabled the proliferation of
aerobic organisms as well as the formation of the ozone layer which blocks
ultraviolet solar radiation,
permitting life on land. Other atmospheric functions important to life
on Earth include transporting water vapor, providing useful gases,
causing small
meteors to burn up before they strike the surface, and moderating temperature.
[108] This last phenomenon is known as the
greenhouse effect:
trace molecules within the atmosphere serve to capture thermal energy
emitted from the ground, thereby raising the average temperature. Water
vapor, carbon dioxide, methane and ozone are the primary
greenhouse gases
in the Earth's atmosphere. Without this heat-retention effect, the
average surface would be −18 °C, in contrast to the current +15 °C, and
life would likely not exist.
[109]
Weather and climate
The Earth's atmosphere has no definite boundary, slowly becoming
thinner and fading into outer space. Three-quarters of the atmosphere's
mass is contained within the first 11 km of the planet's surface. This
lowest layer is called the
troposphere.
Energy from the Sun heats this layer, and the surface below, causing
expansion of the air. This lower-density air then rises, and is replaced
by cooler, higher-density air. The result is
atmospheric circulation that drives the weather and climate through redistribution of thermal energy.
[110]
The primary atmospheric circulation bands consist of the
trade winds in the equatorial region below 30° latitude and the
westerlies in the mid-latitudes between 30° and 60°.
[111] Ocean currents are also important factors in determining climate, particularly the
thermohaline circulation that distributes thermal energy from the equatorial oceans to the polar regions.
[112]
Water vapor generated through surface evaporation is transported by
circulatory patterns in the atmosphere. When atmospheric conditions
permit an uplift of warm, humid air, this water condenses and settles to
the surface as
precipitation.
[110]
Most of the water is then transported to lower elevations by river
systems and usually returned to the oceans or deposited into lakes. This
water cycle
is a vital mechanism for supporting life on land, and is a primary
factor in the erosion of surface features over geological periods.
Precipitation patterns vary widely, ranging from several meters of water
per year to less than a millimeter.
Atmospheric circulation, topological features and temperature differences determine the average precipitation that falls in each region.
[113]
The amount of solar energy reaching the Earth's decreases with
increasing latitude. At higher latitudes the sunlight reaches the
surface at lower angles and it must pass through thicker columns of the
atmosphere. As a result, the mean annual air temperature at sea level
decreases by about 0.4 °C per degree of latitude away from the equator.
[114]
The Earth can be subdivided into specific latitudinal belts of
approximately homogeneous climate. Ranging from the equator to the polar
regions, these are the
tropical (or equatorial),
subtropical,
temperate and
polar climates.
[115]
Climate can also be classified based on the temperature and
precipitation, with the climate regions characterized by fairly uniform
air masses. The commonly used
Köppen climate classification system (as modified by
Wladimir Köppen's student Rudolph Geiger) has five broad groups (humid tropics,
arid, humid middle latitudes,
continental and cold polar), which are further divided into more specific subtypes.
[111]
Upper atmosphere
This view from orbit shows the full Moon partially obscured and deformed by the Earth's atmosphere.
NASA image
Above the troposphere, the atmosphere is usually divided into the
stratosphere,
mesosphere, and
thermosphere.
[108] Each layer has a different
lapse rate, defining the rate of change in temperature with height. Beyond these, the
exosphere thins out into the
magnetosphere, where the Earth's magnetic fields interact with the
solar wind.
[116]
Within the stratosphere is the ozone layer, a component that partially
shields the surface from ultraviolet light and thus is important for
life on Earth. The
Kármán line, defined as 100 km above the Earth's surface, is a working definition for the boundary between atmosphere and space.
[117]
Thermal energy causes some of the molecules at the outer edge of the
Earth's atmosphere to increase their velocity to the point where they
can
escape from the planet's gravity. This causes a slow but steady
leakage of the atmosphere into space. Because unfixed
hydrogen has a low molecular weight, it can achieve
escape velocity more readily and it leaks into outer space at a greater rate than other gasses.
[118] The leakage of hydrogen into space contributes to the pushing of the Earth from an initially
reducing state to its current
oxidizing
one. Photosynthesis provided a source of free oxygen, but the loss of
reducing agents such as hydrogen is believed to have been a necessary
precondition for the widespread accumulation of oxygen in the
atmosphere.
[119]
Hence the ability of hydrogen to escape from the Earth's atmosphere may
have influenced the nature of life that developed on the planet.
[120]
In the current, oxygen-rich atmosphere most hydrogen is converted into
water before it has an opportunity to escape. Instead, most of the
hydrogen loss comes from the destruction of
methane in the upper atmosphere.
[121]
Magnetic field
Schematic of Earth's magnetosphere. The
solar wind flows from left to right
The main part of the
Earth's magnetic field is generated in the Earth's core, the site of a
dynamo
process that converts kinetic energy of fluid convective motion into
electromagnetic energy. The field extends outwards from the core,
through the mantle, and up to the Earth's surface, where it is, to rough
approximation, a
dipole.
The poles of the dipole are presently located close to the Earth's
geographic poles. At the equator of the magnetic field, the magnetic
field strength at the planet's surface is
3.05 × 10−5 T, with global
magnetic dipole moment of
7.91 × 1015 T m3.
[122] The convection movements in the core are chaotic; the magnetic poles drift and periodically change alignment. This causes
field reversals at irregular intervals averaging a few times every million years.
The
most recent reversal occurred approximately 700,000 years ago.
[123][124]
Magnetosphere
The field forms the
magnetosphere, which deflects particles in the
solar wind. The sunward edge of the
bow shock is located at about 13 times the radius of the Earth. The collision between the magnetic field and the solar wind forms the
Van Allen radiation belts, a pair of concentric,
torus-shaped regions of energetic
charged particles. When the
plasma enters the Earth's atmosphere at the magnetic poles, it forms the
aurora.
[125]
Orbit and rotation
Rotation
Earth's rotation period relative to the Sun—its mean solar day—is 86,400 seconds of mean solar time (86,400.0025
SI seconds).
[126] As the Earth's solar day is now slightly longer than it was during the 19th century due to
tidal acceleration, each day varies between 0 and 2
SI ms longer.
[127][128]
Earth's rotation period relative to the
fixed stars, called its
stellar day by the
International Earth Rotation and Reference Systems Service (IERS), is
86,164.098903691 seconds of mean solar time (UT1), or
23h 56m 4.098903691s.[2][n 12] Earth's rotation period relative to the
precessing or moving mean vernal
equinox, misnamed its
sidereal day, is
86,164.09053083288 seconds of mean solar time (UT1)
(23h 56m 4.09053083288s) as of 1982.
[2] Thus the sidereal day is shorter than the stellar day by about 8.4 ms.
[129] The length of the mean solar day in SI seconds is available from the IERS for the periods 1623–2005
[130] and 1962–2005.
[131]
Apart from
meteors
within the atmosphere and low-orbiting satellites, the main apparent
motion of celestial bodies in the Earth's sky is to the west at a rate
of 15°/h = 15'/min. For bodies near the
celestial equator,
this is equivalent to an apparent diameter of the Sun or Moon every two
minutes; from the planet's surface, the apparent sizes of the Sun and
the Moon are approximately the same.
[132][133]
Orbit
Earth orbits the Sun at an average distance of about 150 million kilometers every 365.2564 mean solar days, or one
sidereal year.
From Earth, this gives an apparent movement of the Sun eastward with
respect to the stars at a rate of about 1°/day, which is one apparent
Sun or Moon diameter every 12 hours. Due to this motion, on average it
takes 24 hours—a
solar day—for Earth to complete a full rotation about its axis so that the Sun returns to the
meridian.
The orbital speed of the Earth averages about 29.8 km/s (107,000 km/h),
which is fast enough to travel a distance equal to the planet's
diameter, about 12,742 km, in seven minutes, and the distance to the
Moon, 384,000 km, in about 3.5 hours.
[3]
The Moon revolves with the Earth around a common
barycenter
every 27.32 days relative to the background stars. When combined with
the Earth–Moon system's common revolution around the Sun, the period of
the
synodic month, from new moon to new moon, is 29.53 days. Viewed from the
celestial north pole, the motion of Earth, the Moon and their axial rotations are all
counterclockwise.
Viewed from a vantage point above the north poles of both the Sun and
the Earth, the Earth revolves in a counterclockwise direction about the
Sun. The orbital and axial planes are not precisely aligned: Earth's
axis is tilted some 23.4 degrees from the perpendicular to the Earth–Sun plane (the
ecliptic),
and the Earth–Moon plane is tilted up to ±5.1 degrees against the
Earth–Sun plane. Without this tilt, there would be an eclipse every two
weeks, alternating between
lunar eclipses and
solar eclipses.
[3][134]
The
Hill sphere, or
gravitational sphere of influence, of the Earth is about 1.5 Gm or 1,500,000 km in radius.
[135][n 13]
This is the maximum distance at which the Earth's gravitational
influence is stronger than the more distant Sun and planets. Objects
must orbit the Earth within this radius, or they can become unbound by
the gravitational perturbation of the Sun.
Earth, along with the Solar System, is situated in the
Milky Way galaxy and orbits about 28,000
light years from the center of the galaxy. It is currently about 20 light years above the
galactic plane in the
Orion spiral arm.
[136]
Axial tilt and seasons
Due to the axial tilt of the Earth, the amount of sunlight reaching
any given point on the surface varies over the course of the year. This
causes
seasonal change in climate, with summer in the
northern hemisphere
occurring when the North Pole is pointing toward the Sun, and winter
taking place when the pole is pointed away. During the summer, the day
lasts longer and the Sun climbs higher in the sky. In winter, the
climate becomes generally cooler and the days shorter. Above the
Arctic Circle, an extreme case is reached where there is no daylight at all for part of the year—a
polar night. In the
southern hemisphere the situation is exactly reversed, with the
South Pole oriented opposite the direction of the North Pole.
By astronomical convention, the four seasons are determined by the
solstices—the point in the orbit of maximum axial tilt toward or away from the Sun—and the
equinoxes, when the direction of the tilt and the direction to the Sun are perpendicular. In the northern hemisphere,
Winter Solstice occurs on about December 21,
Summer Solstice is near June 21,
Spring Equinox is around March 20 and
Autumnal Equinox
is about September 23. In the Southern hemisphere, the situation is
reversed, with the Summer and Winter Solstices exchanged and the Spring
and Autumnal Equinox dates switched.
[137]
The angle of the Earth's tilt is relatively stable over long periods of time. The tilt does undergo
nutation; a slight, irregular motion with a main period of 18.6 years.
[138] The orientation (rather than the angle) of the Earth's axis also changes over time,
precessing
around in a complete circle over each 25,800 year cycle; this
precession is the reason for the difference between a sidereal year and a
tropical year.
Both of these motions are caused by the varying attraction of the Sun
and Moon on the Earth's equatorial bulge. From the perspective of the
Earth, the poles also migrate a few meters across the surface. This
polar motion has multiple, cyclical components, which collectively are termed
quasiperiodic motion. In addition to an annual component to this motion, there is a 14-month cycle called the
Chandler wobble. The rotational velocity of the Earth also varies in a phenomenon known as length of day variation.
[139]
In modern times, Earth's
perihelion occurs around January 3, and the
aphelion around July 4. These dates change over time due to
precession and other orbital factors, which follow cyclical patterns known as
Milankovitch cycles. The changing Earth–Sun distance causes an increase of about 6.9%
[n 14]
in solar energy reaching the Earth at perihelion relative to aphelion.
Since the southern hemisphere is tilted toward the Sun at about the same
time that the Earth reaches the closest approach to the Sun, the
southern hemisphere receives slightly more energy from the Sun than does
the northern over the course of a year. This effect is much less
significant than the total energy change due to the axial tilt, and most
of the excess energy is absorbed by the higher proportion of water in
the southern hemisphere.
[140]
Habitability
A planet that can sustain life is termed habitable, even if life did
not originate there. The Earth provides liquid water—an environment
where complex organic molecules can assemble and interact, and
sufficient energy to sustain
metabolism.
[141]
The distance of the Earth from the Sun, as well as its orbital
eccentricity, rate of rotation, axial tilt, geological history,
sustaining atmosphere and protective magnetic field all contribute to
the current climatic conditions at the surface.
[142]
Biosphere
A planet's life forms are sometimes said to form a "biosphere". The Earth's biosphere is generally believed to have begun
evolving about
3.5 bya.
[107] The biosphere is divided into a number of
biomes, inhabited by broadly similar plants and animals. On land, biomes are separated primarily by differences in latitude,
height above sea level and
humidity. Terrestrial
biomes lying within the
Arctic or
Antarctic Circles, at
high altitudes or in
extremely arid areas are relatively barren of plant and animal life;
species diversity reaches a peak in
humid lowlands at equatorial latitudes.
[143]
Evolution of life
Highly energetic chemistry is thought to have produced a self-replicating molecule around
4 bya and half a billion years later the
last common ancestor of all life existed.
[144] The development of
photosynthesis
allowed the Sun's energy to be harvested directly by life forms; the
resultant oxygen accumulated in the atmosphere and formed a layer of
ozone (a form of
molecular oxygen [O
3]) in the upper atmosphere.
[107] The incorporation of smaller cells within larger ones resulted in the
development of complex cells called
eukaryotes.
[145] True multicellular organisms formed as cells within
colonies became increasingly specialized. Aided by the absorption of harmful
ultraviolet radiation by the
ozone layer, life colonized the surface of Earth.
[146] The earliest evidences for
life on Earth are
graphite found to be
biogenic in 3.7 billion-year-old
metasedimentary rocks discovered in
Western Greenland[147] and
microbial mat fossils found in 3.48 billion-year-old
sandstone discovered in
Western Australia.
[148][149]
Since the 1960s, it has been hypothesized that severe
glacial action between 750 and
580 mya, during the
Neoproterozoic, covered much of the planet in a sheet of ice. This hypothesis has been termed "
Snowball Earth", and is of particular interest because it preceded the
Cambrian explosion, when multicellular life forms began to proliferate.
[150]
Following the Cambrian explosion, about
535 mya, there have been five
major mass extinctions.
[151] The
most recent such event was
66 mya, when an asteroid impact triggered the extinction of the (non-avian)
dinosaurs and other large reptiles, but spared some small animals such as
mammals, which then resembled
shrews. Over the past
66 myr, mammalian life has diversified, and several million years ago an African ape-like animal such as
Orrorin tugenensis gained the ability to stand upright.
[152]
This enabled tool use and encouraged communication that provided the
nutrition and stimulation needed for a larger brain, which allowed the
evolution of the human race. The
development of agriculture, and then
civilization, allowed humans to influence the Earth in a short time span as no other life form had,
[153] affecting both the nature and quantity of other life forms.
Natural resources and land use
The Earth provides resources that are exploitable by humans for useful purposes. Some of these are
non-renewable resources, such as
mineral fuels, that are difficult to replenish on a short time scale.
Large deposits of
fossil fuels are obtained from the Earth's crust, consisting of coal, petroleum,
natural gas and
methane clathrate.
These deposits are used by humans both for energy production and as
feedstock for chemical production. Mineral ore bodies have also been
formed in Earth's crust through a process of
ore genesis, resulting from actions of erosion and plate tectonics.
[155] These bodies form concentrated sources for many metals and other useful
elements.
The Earth's biosphere produces many useful biological products for humans, including (but far from limited to)
food, wood,
pharmaceuticals, oxygen, and the recycling of many organic wastes. The land-based
ecosystem depends upon topsoil and fresh water, and the oceanic ecosystem depends upon dissolved nutrients washed down from the land.
[156] In 1980, 5,053
Mha (50.53 million km
2) of the Earth's land surface consisted of forest and woodlands, 6,788 Mha (67.88 million km
2) was grasslands and pasture, and 1,501 Mha (15.01 million km
2) was cultivated as croplands.
[157] The estimated amount of irrigated land in 1993 was 2,481,250 square kilometres (958,020 sq mi).
[14] Humans also live on the land by using
building materials to construct shelters.
Natural and environmental hazards
Large areas of the Earth's surface are subject to extreme weather such as tropical
cyclones,
hurricanes, or
typhoons that dominate life in those areas. From 1980 to 2000, these events caused an average of 11,800 deaths per year.
[158] Many places are subject to
earthquakes,
landslides,
tsunamis,
volcanic eruptions,
tornadoes,
sinkholes,
blizzards, floods, droughts,
wildfires, and other calamities and disasters.
Many localized areas are subject to human-made
pollution of the air and water,
acid rain and toxic substances, loss of vegetation (
overgrazing,
deforestation,
desertification), loss of wildlife, species extinction,
soil degradation, soil depletion, erosion, and introduction of
invasive species.
According to the
United Nations, a scientific consensus exists linking human activities to
global warming
due to industrial carbon dioxide emissions. This is predicted to
produce changes such as the melting of glaciers and ice sheets, more
extreme temperature ranges, significant changes in weather and a
global rise in average sea levels.
[159]
Human geography
A
composite picture consisting of
DMSP/OLS ground-illumination data for 2000 placed on a simulated night-time image of Earth.
Cartography, the study and practice of map-making, and
geography,
the study of the lands, features, inhabitants and phenomena on Earth,
have historically been the disciplines devoted to depicting the Earth.
Surveying, the determination of locations and distances, and to a lesser extent
navigation,
the determination of position and direction, have developed alongside
cartography and geography, providing and suitably quantifying the
requisite information.
Earth has reached approximately seven billion human inhabitants as of October 31, 2011.
[161] Projections indicate that the
world's human population will reach 9.2 billion in 2050.
[162] Most of the growth is expected to take place in
developing nations. Human
population density varies widely around the world, but a majority live in
Asia. By 2020, 60% of the world's population is expected to be living in urban, rather than rural, areas.
[163]
It is estimated that only one-eighth of the surface of the Earth is
suitable for humans to live on: three-quarters is covered by oceans,
while half of the land area is either desert (14%),
[164] high mountains (27%),
[165] or other unsuitable terrain. The northernmost permanent settlement in the world is
Alert, on
Ellesmere Island in
Nunavut, Canada.
[166] (82°28′N) The southernmost is the
Amundsen-Scott South Pole Station, in Antarctica, almost exactly at the South Pole. (90°S)
Independent sovereign nations claim the planet's entire land surface, except for some parts of Antarctica and the odd
unclaimed area of
Bir Tawil between Egypt and Sudan. As of 2013, there are
206 sovereign states, including the 193
United Nations member states. In addition, there are 59
dependent territories, and a number of
autonomous areas,
territories under dispute and other entities.
[14] Historically, Earth has never had a
sovereign government with authority over the entire globe, although a number of nation-states have striven for
world domination and failed.
[167]
The
United Nations is a worldwide
intergovernmental organization that was created with the goal of intervening in the disputes between nations, thereby avoiding armed conflict.
[168] The U.N. serves primarily as a forum for international diplomacy and
international law. When the consensus of the membership permits, it provides a mechanism for armed intervention.
[169]
The first "
earthrise" ever seen directly by humans, photographed by astronauts on board
Apollo 8.
The first human to orbit the Earth was
Yuri Gagarin on April 12, 1961.
[170] In total, about 487 people have visited
outer space and reached Earth orbit as of July 30, 2010, and, of these,
twelve have walked on the Moon.
[171][172][173] Normally the only humans in space are those on the
International Space Station. The station's crew, currently six people, is usually replaced every six months.
[174] The furthest humans have travelled from Earth is 400,171 km, achieved during the
Apollo 13 mission in 1970.
[175]
Cultural and historical viewpoint
The standard astronomical symbol of the Earth consists of a cross circumscribed by a circle,
.
[176]
Unlike the rest of the planets in the Solar System, humankind did not
begin to view the Earth as a moving object in orbit around the Sun
until the 16th century.
[177] Earth has often been personified as a deity, in particular a goddess. In many cultures a
mother goddess is also portrayed as a
fertility deity.
Creation myths
in many religions recall a story involving the creation of the Earth by
a supernatural deity or deities. A variety of religious groups, often
associated with
fundamentalist branches of Protestantism
[178] or Islam,
[179] assert that their
interpretations of these creation myths in
sacred texts are
literal truth
and should be considered alongside or replace conventional scientific
accounts of the formation of the Earth and the origin and development of
life.
[180] Such assertions are opposed by the
scientific community[181][182] and by other religious groups.
[183][184][185] A prominent example is the
creation–evolution controversy.
In the past, there were varying levels of belief in a
flat Earth,
[186] but this was displaced by
spherical Earth, a concept that has been credited to
Pythagoras (6th century BC).
[187] Human cultures have developed many views of the planet, including its
personification as a planetary
deity, its shape
as flat, its position as
the center of the universe, and in the modern
Gaia Principle, as a single, self-regulating organism in its own right.
Chronology
Formation
Artist's impression of the birth of the Solar System
The earliest material found in the Solar System is dated to
4.5672±0.0006 billion years ago (bya);
[188] therefore, it is inferred that the Earth must have been formed by
accretion around this time. By
4.54±0.04 bya[32] the primordial Earth had formed. The
formation and evolution of the Solar System bodies occurred in tandem with the Sun. In theory a
solar nebula partitions a volume out of a
molecular cloud by gravitational collapse, which begins to spin and flatten into a
circumstellar disk, and then the planets grow out of that in tandem with the star. A nebula contains gas, ice grains and
dust (including
primordial nuclides). In
nebular theory planetesimals commence forming as
particulate accrues by
cohesive clumping and then by gravity. The assembly of the primordial Earth proceeded for 10–
20 myr.
[189] The Moon formed shortly thereafter, about
4.53 bya.
[190]
The formation of the Moon remains a topic of debate. The
working hypothesis is that it formed by
accretion from material loosed from the Earth after a Mars-sized object, named
Theia,
impacted with Earth.
[191] The model, however, is not self-consistent. In this scenario, the mass of Theia is 10% of the Earth's mass,
[192] it impacts with the Earth in a glancing blow,
[193] and some of its mass merges with the Earth. Between approximately 3.8 and
4.1 bya, numerous
asteroid impacts during the
Late Heavy Bombardment caused significant changes to the greater surface environment of the Moon, and by inference, to the Earth.
Geological history
Earth's atmosphere and oceans formed by
volcanic activity and
outgassing that included
water vapor. The
origin of the world's oceans was condensation augmented by water and ice delivered by
asteroids,
proto-planets, and
comets.
[194] In
this model, atmospheric "
greenhouse gases" kept the oceans from freezing while the newly forming Sun was only at 70%
luminosity.
[195] By
3.5 bya, the
Earth's magnetic field was established, which helped prevent the atmosphere from being stripped away by the
solar wind.
[196] A
crust formed when the molten outer layer of the planet Earth cooled
to form a solid as the accumulated water vapor began to act in the atmosphere. The two models
[197] that explain land mass propose either a steady growth to the present-day forms
[198] or, more likely, a rapid growth
[199] early in Earth history
[200] followed by a long-term steady continental area.
[201][202][203]
Continents formed by
plate tectonics, a process ultimately driven by the continuous loss of heat from the earth's interior. On
time scales lasting hundreds of millions of years, the
supercontinents have formed and broken up three times. Roughly
750 mya (million years ago), one of the earliest known supercontinents,
Rodinia, began to break apart. The continents later recombined to form
Pannotia, 600–
540 mya, then finally
Pangaea, which also broke apart
180 mya.
[204]
The present pattern of
ice ages began about
40 mya and then intensified during the
Pleistocene about
3 mya. High-
latitude regions have since undergone repeated cycles of glaciation and thaw, repeating every 40–
100000 years. The last continental glaciation ended 10,000 years ago.
[205]
Predicted future
Estimates on how much longer the planet will be able to continue to support life range from
500 million years (myr), to as long as
2.3 billion years (byr).
[206][207][208]
The future of the planet is closely tied to that of the Sun. As a
result of the steady accumulation of helium at the Sun's core, the
star's total luminosity will slowly increase. The luminosity of the Sun will grow by 10% over the next
1.1 byr and by 40% over the next
3.5 byr.
[209]
Climate models indicate that the rise in radiation reaching the Earth
is likely to have dire consequences, including the loss of the planet's
oceans.
[210]
The Earth's increasing surface temperature will accelerate the
inorganic CO2 cycle, reducing its concentration to levels lethally low for plants (
10 ppm for
C4 photosynthesis) in approximately 500-
900 myr.
[206]
The lack of vegetation will result in the loss of oxygen in the
atmosphere, so animal life will become extinct within several million
more years.
[211] After another billion years all surface water will have disappeared
[207] and the mean global temperature will reach
70 °C[211] (
158 °F). The Earth is expected to be effectively habitable for about another
500 myr from that point,
[206] although this may be extended up to
2.3 byr if the nitrogen is removed from the atmosphere.
[208] Even if the Sun were eternal and stable, 27% of the water in the modern oceans will descend to the
mantle in one billion years, due to reduced steam venting from mid-ocean ridges.
[212]
The Sun, as part of its
evolution, will become a
red giant in about
5 byr. Models predict that the Sun will expand to roughly 1 AU (150,000,000 km), which is about 250 times its present radius.
[209][213]
Earth's fate is less clear. As a red giant, the Sun will lose roughly
30% of its mass, so, without tidal effects, the Earth will move to an
orbit 1.7 AU (250,000,000 km) from the Sun, when the star reaches its
maximum radius. The planet was, therefore, initially expected to escape
envelopment by the expanded Sun's sparse outer atmosphere, though most,
if not all, remaining life would have been destroyed by the Sun's
increased luminosity (peaking at about 5,000 times its present level).
[209] A 2008 simulation indicates that the Earth's orbit will decay due to
tidal effects and drag, causing it to enter the red giant Sun's atmosphere and be vaporized.
[213] After that, the Sun's core will collapse into a
white dwarf, as its outer layers are ejected into space as a
planetary nebula.
The matter that once made up the Earth will be released into
interstellar space, where it may one day become incorporated into a new
generation of planets and other celestial bodies.
Moon
Characteristics
| Diameter |
3,474.8 km |
| Mass |
7.349×1022 kg |
| Semi-major axis |
384,400 km |
| Orbital period |
27 d 7 h 43.7 m |
Details of the Earth–Moon system. Besides the radius of each object, the radius to the Earth–Moon
barycenter is shown. Photos from
NASA. Data from
NASA. The Moon's axis is located by
Cassini's third law.
The Moon is a relatively large,
terrestrial,
planet-like satellite, with a diameter about one-quarter of the
Earth's. It is the largest moon in the Solar System relative to the size
of its planet, although
Charon is larger relative to the
dwarf planet Pluto. The natural satellites orbiting other planets are called "moons" after Earth's Moon.
The gravitational attraction between the Earth and Moon causes
tides on Earth. The same effect on the Moon has led to its
tidal locking:
its rotation period is the same as the time it takes to orbit the
Earth. As a result, it always presents the same face to the planet. As
the Moon orbits Earth, different parts of its face are illuminated by
the Sun, leading to the
lunar phases; the dark part of the face is separated from the light part by the
solar terminator.
Due to their
tidal interaction,
the Moon recedes from Earth at the rate of approximately 38 mm a year.
Over millions of years, these tiny modifications—and the lengthening of
Earth's day by about 23
µs a year—add up to significant changes.
[214] During the
Devonian period, for example, (approximately
410 mya) there were 400 days in a year, with each day lasting 21.8 hours.
[215]
The Moon may have dramatically affected the development of life by moderating the planet's climate.
Paleontological evidence and computer simulations show that Earth's axial tilt is stabilized by tidal interactions with the Moon.
[216] Some theorists believe that without this stabilization against the
torques
applied by the Sun and planets to the Earth's equatorial bulge, the
rotational axis might be chaotically unstable, exhibiting chaotic
changes over millions of years, as appears to be the case for Mars.
[217]
Viewed from Earth, the Moon is just far enough away to have almost the same apparent-sized disk as the Sun. The
angular size (or
solid angle)
of these two bodies match because, although the Sun's diameter is about
400 times as large as the Moon's, it is also 400 times more distant.
[133] This allows total and annular
solar eclipses to occur on Earth.
The most widely accepted theory of the Moon's origin, the
giant impact theory, states that it formed from the collision of a Mars-size
protoplanet called
Theia
with the early Earth. This hypothesis explains (among other things) the
Moon's relative lack of iron and volatile elements, and the fact that
its composition is nearly identical to that of the Earth's crust.
[218]
Scale representation of the relative sizes of, and average distance between, Earth and the Moon
Asteroids and artificial satellites
Earth has at least five
co-orbital asteroids, including
3753 Cruithne and
2002 AA29.
[219][220] A
trojan asteroid companion,
2010 TK7, is librating around the leading
Lagrange triangular point, L4, of Earth in
Earth's orbit around the
Sun.
[221][222]
As of 2011, there are 931 operational, man-made
satellites orbiting the Earth.
[223] There are also inoperative satellites and over 300,000 pieces of
space debris. Earth's largest artificial satellite is the
International Space Station.
